Dr. James White did an excellent job of refuting a Muslim named Ijaz Ahmad; his claims about John 20:28 and 9:38.
Even Paul Williams, another Muslim, wrote: “James White demolishes textual corruption claims by Ijaz”
On John 9:38, this is a well known textual variant. It is discussed in Philip Comfort’s book, (which Dr. White recommends) – New Testament Text and Translation Commentary
See also here on the Reliability of the New Testament Text.